Facile synthesis of hierarchical mesopore-rich activated carbon with excellent capacitive performance

Accepted Manuscript Facile synthesis of hierarchical mesopore−rich activated carbon with excellent capacitive performance Dongdong Zhang, Chong He, Jianghong Zhao, Jianlong Wang, Kaixi Li PII: DOI: Reference:

S0021-9797(19)30357-1 https://doi.org/10.1016/j.jcis.2019.03.059 YJCIS 24780

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

21 January 2019 16 March 2019 18 March 2019

Please cite this article as: D. Zhang, C. He, J. Zhao, J. Wang, K. Li, Facile synthesis of hierarchical mesopore−rich activated carbon with excellent capacitive performance, Journal of Colloid and Interface Science (2019), doi: https:// doi.org/10.1016/j.jcis.2019.03.059

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Facile synthesis of hierarchical mesopore−rich activated carbon with excellent capacitive performance Dongdong Zhang, a,b Chong He, b Jianghong Zhao, c Jianlong Wang, a, b* Kaixi Li a, b* a

Institute of Coal Chemistry, Chinese Academy of Sciences, 27 Taoyuan South Road,

Taiyuan 030001, China b

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 010049, China c

Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi

University, 92 Wucheng Road, Taiyuan 030006, China

*Corresponding author Email addresses: [email protected], [email protected]

Abstract

Mesoporous carbons attract increasing attention owing to their potential applications in supercapacitors. So far, controlled synthesis of mesoporous carbons with a narrow pore size distribution relies largely on the complicated template methods. To avoid the use of templates, a surfactant–free emulsion polymerization method is presented for the fabrication of a melamine–modified phenolic resin microrod (MPRR) assembled by micron−sized spherical cells and thin walls. In addition, one–step KOH activation strategy is adopted to synthesize hierarchical mesoporous activated carbon with 2~10 nm narrow mesopores by using MPRR as carbon precursors. The as–prepared mesoporous activated carbon has a high specific surface area of about 2758 m2 g-1 and a mesopore volume of 0.54 cm3 g-1 (calculated by density functional theory), comprising ~43.5 % of total pore volume (~1.43 cm3 g-1). Hierarchical mesopores can significantly accelerate ion transfer and increase micropore accessibility, which endow the carbon with high specific capacitance equal to 409 F g-1 at 0.1 A g-1 and 268 F g-1 at 100 A g-1 in 6M KOH electrolyte, with a high capacitance retention of 66%. Moreover, the assembled symmetric supercapacitor also exhibits good cycling stability in KOH electrolyte and delivers high power density equal to 12080 W kg-1 when energy density is 5.02 Wh kg-1. This finding provides an insight into directional tailoring of mesoporous structures of phenolic resin–based carbon materials at the molecular level for high–performance supercapacitors.

Keywords: Phenolic resin; Emulsion polymerization; Mesoporous activated carbon; Supercapacitor

1. Introduction Supercapacitors, which are high−potential energy storage devices, have attracted considerable interest from both academia and industries because of its rapid charge– discharge cycle, high power density, long cycling lifetime, and applicability in consumer electronics, forklifts, regenerative braking of new energy automobiles, and many others [1–4]. On the basis of electric double-layer capacitances (EDLCs), carbon materials have been outfitted in commercial supercapacitors, owing to its low cost, good conductivity, excellent chemical stability, high specific surface area, and adjustable pore size distribution [5, 6]. Significant amounts of effort have been directed toward the synthesis of carbon materials with engineering microstructures to maximize the electrochemical performance of supercapacitors, such as activated carbons, ordered mesoporous carbons, graphene, carbon nanotubes, carbon nanosheets, carbon fibers, and carbon spheres. In addition, fundamental principles in electrochemistry have also identified that pore size and specific surface area are the most relevant parameters to consider in the electrostatic accumulation of charges at the carbon electrode–electrolyte interface [7–15]. Specifically, mesoporous carbons with 2~10 nm pores are typically regarded as the most brilliant electrode materials because of capacious diffusion pathways, accessible pore volumes, and large exposed surface areas. These properties facilitate ion penetration into micropores to produce EDLCs during the rapid charge–discharge process [12, 16–17]. In general, two primary approaches to design mesoporous carbon have been developed: hard–template and soft–template methods. The former often involves nanocasting, pyrolysis of carbonaceous precursors, and removal of the template, which requires a large amount of time and high costs. The latter can be categorized into self-assembly between

amphipathic block copolymers with polymeric precursors for the formation of orderly molecular aggregates. However, the unstable structure of soft template restricts its efficiency and potential applications [16–20]. To avoid the use of templates and to simplify the synthesis route, directional tailoring of multiple-scale mesoporous structures may be used at the molecular level for increased flexibility. Phenolic resins (PRs) are among the earliest commercial synthetic resins derived from the aldol condensation phenol and formaldehyde with an acidic or alkaline catalyst. Its highly crosslinked and rigid benzene ring structure endow PRs with good thermal stability and high char yield as carbon precursors [21, 22]. However, PR–based porous carbons prepared via multistep carbonization and activation process generate low mesoporous

structures

[23,

24].

Currently,

Fan

et

al.

proposed

one–step

carbonization/activation methods to fabricate porous carbons, with resol as the precursor and KOH solution as the activating agent. The resulting carbons exhibited high specific surface areas and abundant micropores but few mesopores at a low KOH/resol mass ratio. Therefore, those porous carbons showed poor specific capacitance and rate capacity [25]. Similarly, Jaroniec et al. successfully prepared microporous carbon with PR spheres by a modified Stöber recipe and direct KOH activation, but with a considerably small number of mesopores below 4 nm even at high KOH/PR spheres mass ratio [26]. Therefore, the rational design of the microstructure of PRs combined with one-step KOH activation could be an efficient method to design hierarchical mesopore-rich activated carbon. In this study, we propose a simple one–step KOH activation method for the preparation of mesoporous activated carbon (MAC) by using melamine–modified phenolic resin microrod (MPRR). MPRR consisting of micron–sized spherical cells with thin walls is

obtained via emulsion polymerization using ethylene glycol and ethanol as the mixed solvent and melamine–modified thermoplastic phenolic resin as the carbon source. Choosing alcohol with different boiling points can affect the design of multiscale pore– hole structures by the evaporation of ethanol with an increase in polymerization temperature. The alkali–resistant of MPRR is also enhanced via etherification between phenolic hydroxyl and alcoholic hydroxyl and weak conjunction effect between oxygen atom and benzene ring after grafting –CH2CH2OH. These pore–hole structures of MPRR ensure that the molten KOH readily permeated into molecular frameworks, while the alkali–resistant of MPRR guarantees that MPRR exhibits no shrinkage distortion after mixing with KOH. During calcining, KOH is selected as catalyst at a relatively low temperature range because of its ability to react with phenolic hydroxyl and catalyze– activate the C–C bond rupture of methylene−bridged phenols. Subsequently, continuous heating could selectively remove the cleavable fragments, and the potassium compound further activates the carbon to increase micro–mesoporosity. The mesoporous sizes of the developed MAC are well–controlled, ranging from 2 nm to 10 nm, by simply adjusting the heating temperature. Benefiting from the hierarchical rich−mesopores, ion diffusion is accelerated to the micropores and storage, resulting in superior capacitive performance. The optimized MAC-2-700 exhibits high specific capacitance values of 409 F g -1 at a current density of 0.1 A g-1 in 6 M KOH electrolyte, with a considerably high capacitance retention of 66% at 100 A g-1. 2. Experimental 2.1 Materials

Analytically pure KOH, HCl, ethanol, and ethylene glycol were purchased from Tianjin Kermel Chemical Corporation. Heat transfer oil (industrial grade) was obtained from Shanxi Maosheng Chemical Corporation. Melamine-modified thermoplastic phenolic resin (industrial grade) was got from Tianjin Jingnan resin Chemical Corporation. All chemicals were utilized without further purification. 2.2 Synthesis of mesoporous activated carbon 10 g melamine−modified thermoplastic phenolic resin (MPR, melamine was adopted as curing agent) was dissolved in 100 g mixture of ethanol and ethylene glycol with mass ratio of 3:1 and then vigorously stirred until a homogeneous solution A was formed. Subsequently, A was dropwise added into heat transfer oil with stirring of 500 rpm, heated to 120 °C with 1 °C/min. After 2 h of reaction, the mixtures were further filtrated, washed with ethanol until a constant weight was reached, and dried at 120 °C overnight. The melamine−modified phenolic resin microrod (MPRR) was ultimately obtained. 1 g MPRR was immersed into 10 mL KOH solution with a MPRR/KOH mass ratio of 1:2, followed by stirring. The mixture was then dried in vacuum at 80 °C for 12h. The deionized water was added into the above system to dissolve the KOH residue, and the aforementioned steps were repeated until no obvious residue remained. The homogeneous MPRR/KOH was denoted as MPRR@KOH. The mixture was heated at 600 °C, 700 °C, and 800 °C at a ramp rate of 2.5 °C/min and kept 1 h under N2 flow, respectively. The pyrolysis product was washed with 10 wt% HCl and distilled water until pH of 7. The carbon materials were dried at 100 °C for 12 h, denoted as MAC-2-600, MAC-2-700, and MAC-2-800. As a comparison, MPRR was directly carbonized at 700 °C, denoted as C-700.

2.3 Characterization Scanning electron microscope (SEM) observation was performed on JEOL JSM-700 microscope at an acceleration voltage of 10.0 kV. TEM image were taken on FEI Tecnai G2 F20 S-Twin. X–ray diffraction (XRD) was recorded at room temperature using Bruker D8 X–ray diffractometer with Cu Kα radiation (λ=0.15406 nm). Raman spectra was recorded on a Nanofinder 3.0 Raman spectrometer (Tokyo Instrument) using a visible laser beam of 488 nm as the excitation source. Thermogravimetric/derivative thermogravimetric (TG/DTG) measurements were conducted with Ta Q50 from room temperature to 800 °C with a heating rate of 10 °C/min under N 2 atmosphere. Before N2 adsorption measurement, all samples were degassed under vacuum < 10-4 Pa at 473 K for 12 h, and it was taken on Micromeritics ASAP2020 analyzer at −196 °C. The pore size distribution (PSD) was derived from the density functional theory (DFT) method. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) model. The micropore specific surface area, mesopore specific surface area, mesopore cumulative volume and total cumulative volume were calculated by DFT method. X–ray photoelectron spectroscopy (XPS) data was collected on an AXIS Ultra DLD spectrometer with an excitation source of Mg Kα (1486.6 eV). Elemental analysis was carried out using an Elementar Vario Macro EL Cube microanalyzer. The MPRR@KOH was prepared according to the 2.2 steps. 0.1 g MPRR and 0.2 g KOH was annealed at 100 °C, 200 °C, and 300 °C at a ramp rate of 2.5 °C/min and rapidly cooled to room temperature, respectively. The obtained products were washed with distilled water until pH of 7 and freeze–dried for next Fourier transform infrared (FTIR) measurement. FTIR spectroscopy was performed on a Jasco 4000 FTIR spectrometer in attenuated total

reflectance (ATR) mode using a one reflection ATR accessory with ZnSe crystal over the wavenumber range of 4000–400 cm−1. 2.4 Electrochemical measurement MAC, carbon black and binder poly (tetrafluoroethylene) with the mass ratio of 8:1:1 was homogeneous blended by adding moderate ethanol and used for next step. To evaluate the electrochemical behavior of MAC, there–electrode system was adopted in 6 M KOH aqueous electrolyte. The 2.5 mg above mixture was uniformly coated on nickel foam (1×1 cm) and dried at 100 °C overnight as work electrode. Pt foil and Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. For two–electrode cell, two pieces of MAC based electrodes with mass loading 5 mg (ϕ = 10 mm) were symmetrically assembled with 6.0 M KOH as electrolyte and polypropylene as the separator. All the electrochemical measurements were carried out using CHI 660E workstation at 25 °C. Cyclic voltammetry (CV) was measured at scan rates from 10 to 500 mv s-1. Electrochemical impedance spectroscopy (EIS) was recorded in the frequency range from 100 kHz to 0.01 Hz with amplitude of 5 mV. Galvanostatic charge/discharge (GCD) were performed at different current densities from 0.1 to 100 A g -1 and used to calculate the specific capacitance. For the there–electrode system, the gravimetric specific capacitances Cs (F g-1) of the samples were calculated using equation (1): (1) For the two–electrode cell, the capacitance for a single electrode Csingle (F g-1), was calculated based on equation (2):

(2) where i is the discharge current (A), dV/dt is the slope of the discharge curve after ohmic drop (V s-1), and m is the mass of active material of single electrode. The energy density E (Wh kg-1) and power density P (W kg-1) were calculated based on the dates of two electrode cell: (3) (4) where Δ is the cell voltage after ohmic drop (V) and Δ is the discharge time (s).

3. Results and discussion 3.1 Structural evolution of resin to MAC A schematic illustration for preparing MAC is shown in Figure 1a, and the detailed operating procedure is discussed in the Experimental section. The strong effect of hydrogen bonding between MPR phenolic hydroxyl group and alcoholic hydroxyl group led to the capsulation of alkanol in resin matrix and formation of a transparent colloidal solution [27]. When the solution was injected into heat-transfer oil with vigorous stirring, MPRR was formed and stabilized by the strong π–π interactions among benzenes of MPR and heat transfer oil at the alkanol/oil interface with surfactant–free [28]. Ethanol (with a low boiling point of 78.3 °C) began to boil and gradually overflowed from the polymer skeleton to form hollow holes after temperature reached 120 °C. Ultimately, MPRR with pore–hole structures was successfully prepared by a simple emulsion polymerization. When MPRR and KOH were mixed homogeneously, KOH was incorporated into the porous structures of resin, which was facilitated by the strong intermolecular interaction

between alkali and residual alcohol solvents. The molecules of MPRR mainly consist of phenol chain units linked to each other by methylene via aldol condensation [29, 30]. In this process, ethylene glycol incorporated into resin can effectively enhance the alkali– resistant of MPRR (seen in Figure S1, the macro–appearance of MPRR@KOH annealed at 300 °C does not markedly change visually compared with MPRR), thus providing a stable matrix for the subsequent reverse base–catalyzed aldol reaction. Therefore, KOH not only acts as a carbon activator at high temperature but also as an effective catalyst below 300 °C. KOH generally has selective broken C−C bond of methylene–bridge and C−O bond of phenolic hydroxyl at certain temperatures. The detailed mechanism of depolymerization is described in Figure 1b. In the KOH medium, potassium phenolic resinate is obtained after deprotonation of phenolic hydroxyl. The unshared electron pair in oxygen then gravitates toward benzene ring and then forms a benzene radical anion because of conjugation effect, triggering the cleavage of resin chain structure. The fractured small molecules then overflow from resin matrix. As the temperature increases to 400 °C, KOH is reacted with carbon or decomposed according to reactions (1-2). The as–formed solid K2CO3 and K2O lack reactivity below 600 °C. Further, the reactions between potassium alkaline substances (K2CO3 and K2O) and edge carbons of the initial carbon skeleton start with solid–solid or liquid–solid reactions of (5) and (7) at about 700 °C. Meanwhile, the gaseous water and CO2 can penetrate easily into the carbon skeleton and etch carbon atoms (reactions (3) and (6)). Metallic K intercalation accompanying carbon lattice expansion also occurs at the aforementioned temperature. When the temperature is about 800 °C, the activation reactions become more active.

Therefore, these reactions can benefit to increase the mesopore size and improve the microporosity [23, 31–33]. 6KOH +2C → 2K + 3H2 + 2K2CO3

(1)

2KOH → K2O + H2O

(2)

H2O + C → H2 + CO

(3)

K2CO3 → K2O + CO2

(4)

K2CO3 + 2C → 2K + 3CO

(5)

CO2 + C → 2CO

(6)

2K2O + 2C→ 4K + 2CO

(7)

Figure 1. (a) Schematic illustration of the preparation for MAC; (b) Mechanism model of the reverse aldol reaction. To gather more detailed information on the one–step KOH activation process, TG and DTG analyses of MPR, MPRR, and MPRR@KOH were conducted to monitor the onset of their thermal decomposition characteristics. As shown in Figure 2a, weight loss of 46.9 % for the MPR is well below that of the MPRR (82.2%), which may be attributed to the removal of a large amount of ethylene glycol with a higher boiling point (197.3 °C) [28, 34]. Simultaneously, weight loss of 41.5% in MPRR@KOH is observed, and the main residue consists of potassium-containing compounds derived from incipient KOH. The combination of TG and DTG curves in Figure 2a–b indicates that the weight loss occurred in two steps for MPRR. The first weight loss of 58.2% from 150 °C to 360 °C is mainly attributed to the release of alkanol molecules into the MPRR and with a slight cracking reaction of volatile molecules, considering the weight loss of 9.15% for the MPR within this temperature range. The latter weight loss of 23.92% from 360 °C to 800 °C corresponds to the thermal decomposition of MPRR, which is lower than that of MPR (37.66%). Incidentally, MPRR shows a lower weight loss rate than that of MPR, which indicates that MPRR has a higher crosslinking degree and thermostability than MPR [35]. Compared with MPRR below 400 °C, MPR and MPRR@KOH exhibit more multiple–hump functions on DTG curves, demonstrating the occurrence of a more complex pyrolytic reaction. The fraction of weight loss at temperatures ranging from 25 °C to 125 °C corresponds to the volatilization of free small molecules. MPRR@KOH shows a higher rate of weight loss than that of MPR, which can be attributed to enhanced reverse base–catalyzed aldol reaction. With an increase in temperature to 125 °C–200 °C,

a prominent peak is observed on the DTG curves of MPR and MPRR@KOH. The non− crosslinked functional groups of MPR and the fractured bond of KOH−treated MPRR are unstable and decomposed under this temperature range because of the MPRR with no apparent weight loss peak. Moreover, MPRR@KOH exhibits a bimodal weight loss at 200 °C–360 °C. The former peak at 232.5 °C is ascribed to the instant release of ethylene glycol solvent in resin matrix, with MPRR at 289.1 °C. This result may reflect the homogenized decomposition of resin structures from the inside to the outside, which also provides more pathways for the release of unetherified ethylene glycol at a relatively low temperature. The latter peak at 277.3 °C corresponds to the volatilization of etherified ethylene glycol. MPR with no weight loss at 200 °C–360 °C confirms the capsulation of ethylene glycol in MPRR. Ethylene glycol also provides flexible rooms for KOH injection. With a further increase in temperature, MPRR@KOH shows no conventional variation in weight loss peak, compared with MPR and MPRR. These results, combined with the TG data, can be attributed to the solid–solid or liquid–solid reactions between KOH and carbon. The aforementioned analysis suggests that KOH selectively fractured C–C bond between benzene rings bridged by methylene, as well as C–O bond of phenolic hydroxyl and ether below 360 °C. KOH further reacts with carbon atoms and tailors pore structures at higher temperatures. The resin was subjected to FTIR, and carbon was subjected to XPS to detect chemical changes in the intermediate structures and surfaces. FTIR spectra of MPR, MPRR, and MPRR@KOH annealed at 100 °C, 200 °C, and 300 °C are presented in Figure 2c. The elaborate IR characteristic bands and assignments of the samples are listed in Table S1. Typically, for MPR, peaks of the 1,3,5–s–triazine ring (1508, 812 cm-1), C–N (1101 cm-1),

and free –NH2 (1002 cm-1) confirm the presence of melamine grafted on novolak resins [36–38]. After crosslinking and curing, weaker peaks of melamine are observed in the MPRR. Beyond that, peaks at 761 and 669 cm-1 assigned to the out–of–plane deformation vibrations of the aromatic C–H groups decrease significantly from MPR to MPRR. This decrease may be attributed to the condensation reaction between melamine and aromatic ring during emulsion polymerization [39]. C–O stretching vibration of Ar–OH at 1234 cm-1 for MPR shifts to 1209 cm-1 for MPRR, which reveals that phenolic hydroxyl groups are prone to etherification with alcohols. Two strong peaks at 2917 and 2854 cm-1 assigned to the symmetric/asymmetric stretching vibration of methylene also indicate the physicochemical entrapment of alcohols [34, 40–41]. With regard to the MPRR with KOH treatment, a considerably slight but perceptible increase in peak intensity is observed at 1714 cm-1 for C=O stretching. The band at 1432 cm-1 associated with CH2 deformation of benzyl group also significantly weakens, which may be associated with the cleavage of methylene−bridge by aldol condensation reverse reaction [39]. Moreover, common characteristics are also observed in fingerprint region below 200 °C. A sharp peak at 755 cm-1 is observed in MPRR@KOH at 300 °C. This peak is attributed to the transition trend from disubstituted to monosubstituted benzene ring owing to the breakage of sidechain alkyl of benzene ring [42]. On the basis of the aforementioned discussion, KOH treatment significantly influences the chemical compositions of MPRR but only slightly affects the appearance of MPRR in optical photograph (Figure S1). The enhanced alkali–resistant of MPRR can be ascribed to weak conjunction between oxygen atom and benzene ring after etherification. It also suppresses the reaction between KOH

and phenolic hydroxyl, and MPRR provides a relatively separate and stable microchamber for subsequent activation [43].

(a)

(b)

(c)

(d)

Figure 2. (a) TG and (b) DTG curves of MPR, MPRR, and MPRR@KOH; (c) FTIR of MPR, MPRR, and MPRR@KOH treated at 100 °C, 200 °C, and 300 °C; (d) C1s high– resolution XPS spectra of MAC-2-600, MAC-2-700, and MAC-2-800. The structural evolution of the MAC derived from MPRR@KOH was further studied using XPS survey spectra. Figure S3 and Table S2 show that the surfaces of MAC mainly consist of carbon, oxygen and contain a small amount of nitrogen from melamine cracking. Details on carbon formation are characterized by high–resolution analysis of the C1s spectrum. The C1s profiles can be briefly deconvoluted into four peaks at 284.6, 285.7, 287.2, and 288.9 eV, referred to as C–C/C=C, C–O, C=O, and COOH,

respectively [44–45]. By using peak differentiation imitating analysis (Figure 2d) and calculating the relative concentrations of surface functional group, carbon species of MAC-2-600 are found to mainly consist of graphitic sp2 hybridized carbons (284.6 eV, 92.98%), a small amount of phenols or ethers (285.7 eV, 7.02%), and no other carbonyl groups. With an increase in temperature to 700 °C and 800 °C, the C1s peak is resolved into four individual peaks, and the relative amount of the graphitic carbon at the border peak of 284.6 eV decreases from 62.82% to 46.19%. Notably, their surface chemistry is altered owing to the violent chemical reaction between carbon and potassium-containing compounds. The oxygen-doping can improve the wettability of carbon materials and provide more pseudocapacitance [23, 44]. 3.2 Microstructural analysis of MAC SEM image of C-700 in Figure 3a shows well−dispersed microrod shapes and micron−sized holes. The cross−sectional SEM of C-700 exhibits abundant interconnected networks among the spherical cells with thin walls shown in Figure 3b. The structure of C-700 also indirectly verifies that MPRR exhibits a similar arrangement [22, 28], rendering it an excellent carrier for KOH diffusion, storage, and reaction. MPRR is cleaved into small and irregular carbon particles, as shown in Figures 3c and Figure S2. This appearance is attributed to the selective breakage of the chemical bond under base catalysis and strong corrosion during KOH annealing. Moreover, MAC inherits the concave contours from initial hole of carbon rod, indicating that MPRR has a better alkali–resistant under annealing. It can also facilitate electrolyte infiltration at a macro perspective and improve electrochemical performance. Furthermore, the surface microstructure of MAC-2-700 was characterized by TEM. As shown in Figure 3d−3e,

highly disordered microstructure and bright small mesopores are observed at the edges of MAC-2-700. The magnified HR–TEM images also present the multiple ribbon-like graphitic segments.

(b)

(a)

Spherical cell Thin wall

holes

10 µm

(c)

100 µm

10 µm

(e)

(d)

50 nm

10 nm

Figure 3. SEM images of (a) C-700, (b) cross−section, and (c) MAC-2-700. TEM images of MAC-2-700 (d) and HR–TEM of MAC-2-700 (e). Nitrogen adsorption–desorption levels were measured to survey the porous texture of MAC-2-600, MAC-2-700, and MAC-2-800. In Figure 4a, all samples show similar

adsorption isotherms in which the first half is a slope, gradually approaching a plateau, revealing the coexistence of abundant micropores and small mesopores [12, 46]. Specifically, as the annealing temperature increases, nitrogen adsorption–desorption isotherm of MAC-2-800 presents a more pronounced hysteresis loop at a relative pressure p/p of 0.4~0.6 attributed to the enlargement of mesopores, while MAC-2-600 and MAC2-700 share the Type I isotherms. Further, as shown in Figure S4a, C-700 also exhibits the typical Type I isotherm representative of microporous carbon with 0.5~1 nm. PSD curve of C-700 also proves the absence of mesopores (Figure S4b). However, the significant improvement in pore size of MAC indicates a regular bimodal distribution with sizes in the 1.0~1.5 and 2~10 nm ranges, and no obvious patterns below 1 nm are observed. These results completely vary from those of C-700 depicted in Figure S4b. Rapid incremental pore volumes in size range from about 1~1.5 nm with an increase in activation temperatures are shown in Figure 4b. In addition, the pore volumes and pore sizes (>2 nm) of MAC tend to increase from MAC-2-600, MAC-2-700, to MAC-2-800, respectively. This finding may be ascribed to the enhancement of chemical reactivity between carbons and H2O, CO2, K2CO3, and K2O (reactions of (3), (5), (6), and (7)), as well as metallic K efficient intercalation into carbon lattices at higher temperature [23]. Those reactions are beneficial for the formation of porosity structures after the removal of K−containing compounds. The pore structure parameters are summarized in Table 1 in which MAC-2-600, MAC-2-700, and MAC-2-800 exhibit increasing SBET values of 1958, 2758, and 3001 m2 g-1, respectively, which is far exceed C-700 equal to 550 m2 g-1. Moreover, the obtained MAC show increased DFT surface areas of 1349, 1818, and 1874 m2 g-1 with an increase in annealing temperature. This behavior indicates that micro–

mesoporous structures and high specific surface areas ascribe to the multiple KOH activation reactions. Meanwhile, MAC-2-700 possesses a higher DFT micropore specific surface area of 1417 m2 g-1 than those of MAC-2-600 (1019 m2 g-1) and MAC-2-800 (1234 m2 g-1). It suggests that MAC-2-700 can provide sufficient active sites for ion adsorption and storage at low current density. With an increase in activation temperature, the DFT mesopore surface area of each of MAC-2-600, MAC-2-700, and MAC-2-800 increases as temperature increases (330, 401, and 712 m2 g-1, respectively). Accordingly, the DFT mesopore volumes of MAC-2-600, MAC-2-700, and MAC-2-800 also increase (0.32, 0.54, and 1.06 cm3 g-1, respectively), comprising 37.6 %, 43.5 %, and 65.0 % of the total pore volume, respectively. These results combined with C-700 data confirm that the etching of carbon atoms on the surface and edge by redox reactions can be used to effectively construct mesopore−rich carbon materials. Furthermore, this mesopore structure can facilitate ion mobility as well as ion adsorption at higher charge–discharge rates. Figure 4c presents XRD patterns of C-700 MAC-2-600, MAC-2-700, and MAC-2-800. The MAC exhibit two broader diffraction peaks ascribed to typical (002) and (100) reflections of amorphous carbon. The (002) peak around 23.5° of C-700 shifts visibly to the high angle about 29° of MAC, indicating that the interlayer spacing d(002) gradually declines according to Bragg’s law. Compared with C-700, the shapes of (002) peak for MAC is relatively strong and faintly narrow, suggesting an increase in the degree of graphitization [47, 48]. However, the shapes of (100) peak of 2θ=42° change to flat from C-700 to MAC. This change reveals a decrease in the dimension of graphene sheet planes, which may be ascribed to the methylene−bridge fission and the immature rearrangement

of aromatic nucleus in lattice. Raman spectra are also obtained to further clarify their structural differences. Two characteristic peaks located at ~1350 and ~1580 cm-1 are presented in Figure 4d, which are ascribed to the D and G bands of activated carbon. D band represents the A1g symmetry lattice vibration of the disordered carbon, edge defect, and other defects, whereas G band is attributed to the E2g symmetry stretching vibration of ordered graphite lattice [44, 47]. The increasing IG/ID intensity ratio from C-700 to MAC also indicates intensified graphitization, which is consistent with the XRD result. Furthermore, the deconvolution of D and G peaks from Raman spectra is taken to reveal the variation in carbon microcrystallites with activation temperature. Raman spectra for MAC-2-600, MAC-2-700, and MAC-2-800 are characterized using four vibration modes labeled as D1, D3, D4, and G, respectively. D1 peak (~ 1340 cm-1) represents the carbon lattice defect caused by heteroatoms doping; D3 peak (~ 1450 cm-1) is ascribed to the disordered carbon; D4 peak (~ 1200 cm-1) reflects the polyenoid structure; G peak (~ 1580 cm-1) corresponds to the graphite aromatic layer [49]. As shown in Figure S5 and Table S4, the intensity of D1 peaks for MAC-2-600, MAC-2-700, and MAC-2-800 decreases because of removing of oxygen or nitrogen. The intensity ratio I D3/IG increases from 0.562 (MAC-2-600) to 0.612 (MAC-2-700) and 0.608 (MAC-2-800). In addition, the position of G peaks for MAC-2-600, MAC-2-700, and MAC-2-800 shifts to lower frequencies and wavenumber. This is close to 1580 cm-1 for perfect crystalline graphite, indicating a slight increase in the degree of order with enhancing activation temperature.

(a)

(c)

(b)

(d) IG/ID=1.056

IG/ID=1.044

IG/ID=0.986

IG/ID=0.967

Figure 4. (a) N2 adsorption–desorption isotherms; (b) PSD curves of MAC-2-600, MAC2-700, and MAC-2-800; (c) XRD patterns; (d) Raman spectra of C-700, MAC-2-600, MAC-2-700, and MAC-2-800.

Table 1. Pore structure parameters of the samples SBET[a]

SDFT[b]

S≤ 1nm[c]

S1-2nm[c]

Smicro [c]

Smeso[d]

V≤1nm[e]

V1-2nm[e]

Vmeso[f]

Vtotal[g]

Vmeso/Vtotal

(m2g-1)

(m2g-1)

(m2g-1)

(m2g-1)

(m2g-1)

(m2g-1)

(cm3g-1)

(cm3g-1)

(cm3g-1)

(cm3g-1)

(%)

C-700

550

567

548

19

567

0

0.19

0.01

0

0.20

0

MAC-2-600

1958

1349

663

356

1019

330

0.21

0.32

0.32

0.85

37.6

MAC-2-700

2758

1818

760

657

1417

401

0.23

0.47

0.54

1.24

43.5

MAC-2-800

3001

1874

645

589

1234

712

0.18

0.39

1.06

1.63

65.0

Sample

[a] BET specific surface area; [b] DFT specific surface area; [c] DFT micropore specific surface area; [d] DFT mesopore specific surface area (2 nm < pore size < 50 nm); [e] DFT micropore volume; [f] DFT mesopore volume (2 nm < pore size < 50 nm); [g] DFT total volume. 3.3 Electrochemical performance With the advantages of hierarchical small mesoporous structure and abundant micropores, MAC show high potential for application in manufacturing supercapacitors. To verify the capacitive response of C-700, MAC-2-600, MAC-2-700, and MAC-2-800, electrochemical performances were measured in a three−electrode system with 6 M KOH as the aqueous electrolyte. CV curves of MAC exhibit a good quasi−rectangular shape at a scan rate of 10 mV s-1 (Figure 5a), corresponding to a typical electrical double−layer capacitive behavior. By contrast, CV shape of C-700 is largely distorted because of the slow ion diffusion dynamics in the micropores [34, 48]. MAC-2-700 clearly displays a larger integral CV area than those of C-700, MAC-2-600, and MAC-2-800, revealing a higher specific capacitance. Even at the scan rate of 500 mV s-1 (Figure 5b), the current density of MAC-2-700 remains higher than those of others, suggesting a higher specific capacitance. This high specific capacitance of MAC-2-700 may be attributed to the higher microporous specific surface area and appropriate fine mesoporous texture, which

provide abundant active adsorption sites and faster ion electro−adsorption responses [50– 52]. Figure 5c shows GCD curves of C-700, MAC-2-600, MAC-2-700, and MAC-2-800 at a current density of 10 A g -1. Notably, MAC-2-700 shares a longer discharge time and higher specific capacitance. In addition, all samples present an obvious IR drop about 0.175, 0.055, 0.044, and 0.055 V for C-700, MAC-2-600, MAC-2-700, and MAC-2-800, respectively. The relevant equivalent series resistance (ESR) values are 6.63, 2.29, 2.20, and 2.55 Ω, respectively. The lower IR drop and ESR of MAC-2-700 render potential increases in power performance and energy efficiency [53]. On the basis of GCD measurements, specific capacitance values as functions of current density were calculated and shown in Figure 5d. The specific capacitance values of MAC-2-600, MAC-2-700, and MAC-2-800 show an irreversible boost within a wide current density from 0.1 A g -1 to 100 A g-1 relative to that of C-700. It suggests that highly developed small mesoporous structures are favorable for efficient charge carrier transport and exposure of electrolyteaccessible surface areas. The specific capacitance of MAC-2-700 is 408.9 F g-1 at a current density of 0.1 A g -1, which is considerably higher than those of C-700 (181.5 F g1

), MAC-2-600 (355.5 F g-1), and MAC-2-800 (309.5 F g-1). However, the specific

capacitance of MAC-2-700 decreases to 320.8 F g-1 at 1 A g-1, which may be ascribed to the reduced contributions of hydrogen evolution and pseudocapacitance [54, 55]. When the charge−discharge current density is increased 1000 times (100 A g -1), it also exhibits superior capacitance equal to 267.9 F g-1, which is higher than those of C-700 (65.2 F g-1), MAC-2-600 (155.9 F g-1), and MAC-2-800 (224.0 F g-1). This finding is enhanced relative to the results reported in the literature (Table S5). Specifically, the capacitance

retention rates of C-700, MAC-2-600, MAC-2-700, and MAC-2-800 are increased from 35.9%, 43.8%, and 65.5%, respectively, to 72.3% (Figure S6a). These results are closely associated with the mesopore size. The increases in mesopore size lead to the enhanced ion rapid transport and micropore accessibility for charge accommodation, consequently increasing capacitance retention under high load. Electrochemical impedance spectroscopy, which records the impedance of carbon electrode as a function of sinusoidal frequency by applying 5 mV alternating potential, is used in for relating kinetics of electrode processes, ion transport resistance, and EDLCs mechanism. Figure 5e presents the Nyquist impedance plots of C-700, MAC-2-600, MAC-2-700, and MAC-2-800. The samples show an inclinable high phase−angle behavior at a low frequency with an increase in KOH activation temperature, which indicate their gradual approach to the quasi−ideal capacitor characteristic [47, 50]. MAC2-800 possesses the largest phase angle that is ascribed to rapid ion diffusion to the interface, fast adsorption−desorption response, and high−efficiency charge storage at larger mesopores and specific surface areas of carbon materials. The semicircles of Nyquist plots at a high frequency characterize the charge transfer interface resistance or electrical conductivity of MAC-based electrode (Figure 5f). Compared with C-700, the reduced radius of MAC is ascribed to a stronger ion penetrated through the carbon matrix and a lower contact resistance at the electrode material/current collector interface. Meanwhile, Z’ intercept of MAC-2-800 is considerably smaller than those of MAC-2600 and MAC-2-700, suggesting its better conductivity. Nyquist plots of MAC also show a 45° slope length at the junction of semicircles and vertical, demonstrating lower Warburg impedance than C-700. The operating frequency is defined as f (in Hz), in

which the real and imaginary parts of impedance are equal between low and high frequencies. It is closely related to the characteristic relaxation time (τ

f) as shown in

Bode plots (Figure S6b). The operating frequencies of C-700, MAC-2-600, MAC-2-700, and MAC-2-800 are 3.38, 0.68, 0.46, and 0.46 Hz, corresponding to the relaxation time constants 0.30, 1.47, 2.17, and 2.17 s, respectively, along with increasing discharge time in this order. On the basis of the aforementioned structural characteristics and electrochemical parameters, mesopore−dominated MAC show higher specific capacitance and rate capability at a high rate than those of the micropore−type C-700. This result reveals that charges are more inclined to be adsorbed and stored as electric double layers in hierarchical micro−mesopores in carbon frameworks. The reason is that the mesopores are favorable to smooth mass transport and accessible to electrolytes, and smaller micropores can be efficiently penetrated by ions during the rapid charge−discharge process. Furthermore, MAC also exhibit small charge transfer resistance and Warburg impedance [56–58]. Comprehensive analysis of various indexes, MAC-2-700 with moderate mesoporous distribution is a good electrode material for high-performance supercapacitors in an aqueous electrolyte.

Figure 5. CV profiles at (a) 10 mV s-1 and (b) 500 mV s-1; (c) GCD curves at current densities from 10 A g-1 ; (d) Specific capacitances of all carbon electrodes as a function of current densities; (e) Nyquist plots; (f) localized Nyquist plots for C-700, MAC-2-600, MAC-2-700, and MAC-2-800.

(a)

(b)

(c)

(d)

Figure 6. (a) CV profiles of MAC-2-700 at scan rate from 10 to 500 mV s-1 ; (b) GCD curves of MAC-2-700 at different current densities from 10 A g-1 to 50 A g-1; (c) Specific capacitance values at different current densities and (d) Ragone plots of MAC-2-600, MAC-2-700, and MAC-2-800. A symmetric supercapacitor was assembled via two identical pieces of MAC electrodes to further evaluate the capacitive performances in 6M KOH. CV profiles of MAC-2-700 (Figure 6a) and MAC-2-800 (Figure S7b) slightly deviate from the ideal rectangular shape at a wide range of 10~500 mV s-1, suggesting their excellent capacitive performance. By contrast, MAC-2-600 (Figure S7a) shows an apparent distortion curve and small CV integral areas. This finding indicates that enlarged mesopores can improve ions diffusion dynamics and electrochemical response. Moreover, an obvious H2 evolution signal can be observed at the low scan rate and 1V for all samples. This phenomenon may be related to some surface defect sites, oxygen, and nitrogen functional

group in MAC, which enhances the electrocatalytic activity and induces hydrogen evolution [55, 59-60]. GCD profiles of MAC-2-700 at higher current densities from 10 A g-1 to 50 A g-1 are depicted in Figure 6b. It exhibits nearly symmetric triangular shapes and slight ohmic drop, indicating a high coulombic efficiency and negligible ESR. Compared with MAC-2-700, MAC-2-600 (Figure S7c) shows a more evident ohmic drop, and MAC-2-800 has a reduction in discharge time, as observed in the GCD curves (Figure S7d). The specific capacitances of MAC-2-600, MAC-2-700, and MAC-2-800 as a function of the current density are compared in Figure 6c. MAC-2-700 delivers a specific capacitance of about 243.5 F g-1 at 1 A g-1, which is higher than that of MAC-2600 (215 F g-1) and MAC-2-800 (190.8 F g-1). However, when the current density reaches 50 A g-1, the specific capacitance of MAC-2-800 (145.1 F g-1) can match that of MAC-2700 (154.5 F g-1), which is higher than MAC-2-600 (133.0 F g-1). It is demonstrated that the large mesopores contribute to charge storage characteristics at high current density. Consequently, MAC-2-800 exhibits a higher rate capability of 76.2%, compared with MAC-2-700 (63.4 %) and MAC-2-600 (62.1 %). This result is in accordance with the three−electrode system (Figure S5a). Figure 6d presents the Ragone plots of MAC recording the relevance between energy density and power density. At a lower power density of 250 W kg-1, MAC-2-700 delivers a maximum gravimetric energy density of 8.45 Wh kg-1, which surpasses the values of MAC-2-600 (7.47 Wh kg-1) and MAC-2-800 (6.63 Wh kg-1). In addition, MAC-2-700 and MAC-2-800 can deliver a high power density of 12080 W kg-1 at the same energy density above 4.70 Wh kg-1. However, the power density of MAC-2-600 is significantly reduced to less than 6550 W kg -1 at low energy density of 1.20 Wh kg-1 derived from the small discharge time (Δ ) and large

ohmic drop during GCD testing. It is indicated that the micropore−dominated MAC-2600 cannot form the maximum electric double layer capacitance because of the lack of adequate mesoporous ion diffusion channels. MAC-2-700 also manifests a superior long−term cycling stability of ~96% after 10,000 cycles at a current density of 5 A g -1 (Figure S7e). The excellent electrochemical performance of MAC-2-700 can be ascribed to the synergistic effects of high BET specific surface area, abundant micropores, superior hierarchical mesoporosity (2~8 nm), and appropriate oxygen defects.

4. Conclusions In summary, an engineered melamine−modified phenolic resin microrod (MPRR) consisting of numerous spherical cells with thin walls is successfully prepared by emulsion polymerization. MPRR with pore–hole structure and alkali–resistant is a preferable medium for molten KOH to easily permeate resin frameworks. Mesoporous activated carbon (MAC) with 2~10 nm tunable pores is obtained by one-step KOH activation. The optimized MAC-2-700 shows an appropriate mesopore volume of 0.54 cm3 g-1, which can moderate diffusion resistance and increase pore accessibility. Meanwhile, benefiting from the abundant micropores, it also has a high specific surface area of about 2758 m2 g-1 to provide more ion absorption sites. Therefore, MAC-2-700 exhibits high specific capacitance equal to 409 F g-1 at 0.1 A g-1 and 268 F g-1 at 100 A g1

in 6M KOH with a superior capacitance retention rate of 66%. The assembled

symmetric supercapacitor based on MAC-2-700 delivers a higher power density of 12080 W kg-1 at an energy density of 5.02 Wh kg-1 and better cycling stability. Moreover, the

as−made mesoporous activated carbons exhibit potential for application in metal−free electrocatalysts, lithium−sulfur batteries, supercapacitors, and CO2 captures.

Appendix A. Supplementary data Photograph, Pore structure, XPS, Raman, and electrochemical performances.

Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. U1510204 and 51672291), the Shanxi Province Coalbased Key Scientific and Technological Project (No. MD2014-09), and the Shanxi Province Key Research and Development Plan (No. 201603D321023). Notes The authors declare no competing financial interest. References [1] S. Zhang, N. Pan, Supercapacitors performance evaluation. Adv. Energy Mater. 5 (2015) 1401401. [2] J. L. Liu, J. Wang, C. H. Xu, H. Jiang, C. Z. Li, L. L. Zhang, J. Y. Lin, Z. X. Shen, Advanced energy storage devices: basic principles, analytical methods, and rational materials design. Adv. Sci. 5 (2018) 1700322. [3] H. Jiang, P. S. Lee, C. Z. Li, 3D carbon based nanostructures for advanced Supercapacitors. Energy Environ. Sci. 6 (2013) 41–53.

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Graphical Abstract